Copending application Ser. No. 554,174, filed Nov. 22, 1983, now abandoned herein incorporated by reference in its entirety, teaches a method for deinking secondary fiber sources, such as wastepaper, by fiberizing the secondary fiber source in a substantially dry state to produce substantially discrete fibers and ink-containing fines and separating the fibers from the fines.
In designing a commercial dry deinking process for processing large volumes of wastepaper as described in the above-mentioned application, it has been found advantageous to provide a consistent and uniform feed rate of shredded wastepaper to the fiberizers. To accomplish this, one might suggest feeding the shredded wastepaper to the fiberizers from a storage/metering bin (hereinafter more fully described) which provides a large material residence time to overcome any upstream fluctuations in material flow rates and at the same time provide a controlled, finely-tuned feed rate to the fiberizers. However, it has been found that in order to provide sufficient metering of the shredded wastepaper to the fiberizers, the outlets of the storage/metering bin must be relatively small. It has also been discovered that if too many oversized pieces of shredded wastepaper enter the storage/metering bin which do not pass through the outlets, the oversized pieces simply recycle within the storage/metering bin and quickly accumulate, causing the storage/metering bin to fill and overflow.
Since wastepaper generally is provided in baled form containing a wide range of sizes and shapes of wastepaper and contaminants, it is necessary to first shred the wastepaper into pieces no larger than a size which is suitable for subsequent processing. Because of the size limitations of the outlets of the storage/metering bin described above, it is therefore necessary to control the size of the shredded wastepaper leaving the shredding device. A screened shredding device, such as a screened hammermill, having screen openings of a size sufficiently small to be compatible with the size of the storage/metering bin outlets would seemingly provide control of the size of the shredded wastepaper sufficient to satisfy the storage/metering bin size limitation. Large pieces are retained within the shredding device until they are reduced to a size small enough to pass through the screen and hence, by design, also small enough to pass through the outlets of the storage/metering bin. Such a shredding device could be, as is common practice, coupled with a shredder fan, positioned just downstream of the shredder screen, which would serve to pull air and entrained materials through the shredding device and screen and propel or airvey the shredded material downstream. The thus shredded and screened wastepaper could then be deposited into the storage/metering bin(s).
However, it has also been discovered that in some instances screened shredders, such as screened hammermills, allow wadded-up wastepaper to pass through their screens if the wadded-up wastepaper has a sufficiently small 2-dimensional size. The wadded-up wastepaper, such as is commonly found in office wastebaskets, is initially present in the baled feed material and, if wadded-up tightly enough, can pass through the shredder virtually unaltered. Surprisingly, these wadded-up materials are thereafter "opened-up" by the blade action of the shredder fan which, as previously mentioned, is used to draw the feed material through the shredders and propel the shredded material downstream. Some of these opened-up pieces of wastepaper are larger (2-dimensionally) than the openings in the shredder screen through which they passed in a wadded form and are also too large to exit the storage/metering bin. This situation can cause operational problems with the storage/metering bin as previously mentioned.
The foregoing discoveries led to the solution of screening the shredded material a second time, but at some point in the process after the shredder fan and prior to the storage/metering bins to remove any opened-up pieces of wastepaper which would have been too large to pass through the openings in the shredder screen or the outlets of the storage/metering bin(s). The need for an additional screening step after the shredder screen is unexpected since the shredder screen could be expected to be sufficient. However, due to the peculiar nature of wastepaper as a feedstock, most notably its flexibility, low density, and wide variety of shapes and sizes, which allows wadded-up wastepaper to open up or "expand" to a larger 2-dimensional size, a second screening is necessary. This problem is not present when processing other materials, such as wood chips, for which storage/metering bins are designed. Hence because of the novelty of the dry deinking process, it has become necessary to invent ways to handle wastepaper in preparation for dry deinking